The present invention relates to a multilayer film which may serve as a protective carrier sheet for metal foil and a metal foil bearing laminate having special utility in the production of printed circuit boards. More particularly, it relates to a multilayer film which is useful as a temporary, peelable protective carrier sheet for a cladding metal foil used in the production of printed circuit boards.
Various methods are known for producing metal clad laminates useful in the production of printed circuit boards. In the typical process, metal clad laminates are produced by impregnating a fibrous substrate with a resin varnish, drying the substrate to prepare a prepreg, stacking one or more prepregs to a desired thickness, bonding a cladding metal foil on one or both sides of the prepreg stack with, optionally, an adhesive coating being applied on the surface of the metal foil, and finally curing the assemblage under heat and pressure in a molding press to laminate/mold it. Examples of patents which discuss various aspects of this conventional process include U.S. Pat. No. 4,302,501 directed to porous, heat resistant insulating substrates for use as prepregs in the production of printed circuit boards and U.S Pat. No. 4,410,388 directed to an aging process to avoid variations in performance of metal clad laminates in the production of printed circuit boards.
A recognized problem in the production of printed circuit boards is that the cladding metal foil may be subject to damage in the molding press. For that reason it has been suggested that a protective pad be used between the molding press and the metal foil layer. For example in U.S. Pat. No. 4,690,845 there is disclosed a "stratiform" sheet useful in a flat press method of laminating printed circuit boards. The stratiform sheet has a thermoplastic layer, a polymer release layer on one side, a polymeric stabilizing layer having a melting point higher than the laminating temperature peak, and a melting point higher than the peak. The stratiform sheet is said to be characterized by substantial freedom from creases, trapped gases, and contaminants. The stratiform sheet is utilized in the method of laminating printed circuits in a flat press by providing a work piece comprising one or more circuit assemblies in need of lamination, providing the stratiform sheets, and arranging each circuit assembly in a book with one of said stratiform sheets on each side of a circuit assembly with the release layers thereof facing the circuit assemblies in the press, then pressing the book for the press cycle time appropriate to the product and thickness of the book.
It is also known to use a poly-4-methylpentene-1 film as a releasable protective layer between a stainless steel press and a copper cladding metal foil in the manufacture of copper clad laminates. See Japanese patent document number 58/128,846 dated Aug. 1, 1983 in the name of Toshiba Chem. KK. See also Japanese patent documents number 57/070,654 and 57/070,653 dated May 1, 1982 in the name of Mitsubishi Gas Chem. Ind. (releasing film comprising 4-methylpentene-1 homopolymer, copolymer or a mixture with other resins used in laminating prepregs) and Japanese patent document number 56/111,637 dated Sept. 3, 1981 in the name of Hitachi Chemical KK (release film comprising 4-methylpentene-1homopolymer, and copolymers with polyethylene, polypropylene, vinyl acetate, etc. used in laminated sheet manufacture).
A somewhat similar system is shown in U.S. Pat. No. 4,753,847 where there is disclosed a mold release sheet which can be used in the production of printed circuit boards. The mold release sheet comprises a film of polyester, nylon or cellulose acetate treated for promoting resin adhesion and having a coating of a thin release layer of a cured release acrylated oligomer resin on at least one side of the film. In the molding operation, the mold release sheet may be selectively interleaved among the laminate-making layers and the press platens and caul plates.
The stratiform sheet/mold release sheet systems of the Japanese patent documents and U.S. Pat. Nos. 4,690,845 and 4,753,847 have a number of advantages. For example, at Col. 4, lines 45-60 of U.S. Pat. No. 4,753,847 the following advantages are listed:
The release sheet structure:
(i) does not stick to the clad, the prepreg sheet, the kraft sheet nor the laminate surfaces; PA0 (ii) assists in preventing migration of flowable material from one laminate component to another; PA0 (iii) does not shrink; PA0 (iv) is resistant to embrittlement; PA0 (v) has good tensile strength; PA0 (vi) has sufficient rigidity to assist in handling; PA0 (vii) does not emit excessive odors; PA0 (viii) minimizes the static electric charges, thus easing removal from the mold; and PA0 (ix) has good hole-forming characteristics whether drilled or punched.
Despite these advantages, a problem remains. While the stratiform sheet/mold release sheet protects the cladding metal foil during the molding process, the cladding metal foil may be subject to damage while handling before, during, or after the molding process. Typically, the cladding metal foil is a thin foil such as a copper foil which must be kept intact and free from surface contamination. It would, therefore, be desirable to protect/support the cladding metal foil particularly during handling before molding.
There is in the patent literature at least one reference to use of a transfer/support for a copper film to be used in producing printed circuit boards. Lifshin et al in U.S. Pat. No. 4,455,181 discloses vapor depositing a film of zinc on a copper film on a silica-coated aluminum carrier sheet, vapor depositing a silica film on the resulting zinc-copper foil, bonding the resulting body to a substrate (prepreg) and then stripping the silica-coated aluminum carrier sheet from the copper-clad laminate. At col. 1, lines 22-35, it is disclosed that rather than an aluminum carrier sheet, the carrier sheet may be a "sheet material of other metals as well as of plastics, such as DuPont commercial products known as MYLAR and KAPTON and other organic polymeric materials of similar flexibility which will withstand the processing temperatures involved in this invention and have the strength at the temperature of deposition of the copper film and the characteristics of inertness and bondability to release agent coatings necessary for coating adherence as copper-clad laminate products are stripped from the carrier sheets."
It would clearly be desirable to use an organic polymeric material as the carrier sheet in view of the lower cost, easier handling, more flexible nature of such materials. However, a number of difficulties are encountered in that regard. Foil contamination due to resin additives exuding to the foil surface must be controlled. Excessive adhesion also results in polymer contamination of the foil. Likewise, adhesion of the backside of a polymeric carrier to the molding press during molding is also a problem. The polymeric carrier sheet must provide sufficient support for the cladding metal foil so that the high temperatures and molding pressures do not damage it. Finally, the carrier sheet must be easily strippable from the cladding metal foil (i.e. it must have a peel value of less than about 0.4 lbs./inch-width). This is difficult to achieve with a polymer carrier because of polymer degradation during molding (which has been found to be increased due to the catalytic effect of the copper foil) and migration during molding of any adhesives used in conjunction with the polymeric carrier.
Accordingly, the need exists for an improved polymeric protective carrier sheet for cladding metal foil and for a cladding metal foil having a carrier sheet releasably laminated thereto which has special utility in the production of printed circuit boards.